[CRT] Fix _matherror and _setusermatherror

We previously used 2 different versions: one from wine and one from mingw-w64.
The former was used in msvcrt, the latter was statically compiled into the executable. When using MS libs, there is only one _matherr, which is statically linked into the executable and does nothing (it's not really a function for users to be called).
_setusermatherror should only exist in msvcrt and not statically, which wouldn't work at all.

sdk/lib/crt/math/math.cmake

@@ -3,6 +3,7 @@ list(APPEND LIBCNTPR_MATH_SOURCE
     math/abs.c
     math/div.c
     math/labs.c
+    math/usermatherr.c
 )
 
 if(ARCH STREQUAL "i386")

sdk/lib/crt/math/usermatherr.c

@@ -0,0 +1,36 @@
+/*
+ * PROJECT:     ReactOS CRT library
+ * LICENSE:     MIT (https://spdx.org/licenses/MIT)
+ * PURPOSE:     Implementation of __setusermatherr and _invoke_user_matherr
+ * COPYRIGHT:   Copyright 2021 Timo Kreuzer <timo.kreuzer@reactos.org>
+ */
+
+// DO NOT SYNC WITH WINE OR MINGW32
+
+#include <math.h>
+
+/* MS headers have this in corecrt_startup.h */
+typedef int (*_UserMathErrorFunctionPointer)(struct _exception *);
+
+static _UserMathErrorFunctionPointer user_matherr = NULL;;
+
+void
+__cdecl
+__setusermatherr(_UserMathErrorFunctionPointer func)
+{
+    user_matherr = func;
+}
+
+int
+__cdecl
+_invoke_user_matherr(struct _exception *e)
+{
+    if (user_matherr != NULL)
+    {
+        return user_matherr(e);
+    }
+    else
+    {
+        return 0;
+    }
+}